A jailed member of Russia’s opposition has said he fears for his life after the death of Alexei Navalny, as the Putin critic’s widow, Yulia Navalnaya, demanded the Kremlin release his body so he can be “buried with dignity”. Yet as the appeals came, the Russian state opened a new criminal case on Tuesday against Navalny’s brother, Oleg, signalling it would continue the pressure on his family and supporters as they seek to mourn the late opposition leader. Navalnaya has publicly accused Vladimir Putin of ordering the murder of her late husband and said investigators were retaining his body in order to cover up a political assassination. The national security spokesperson John Kirby on Tuesday said the Biden administration would announce a “major sanctions package” on Friday to “hold Russia responsible for what happened to Navalny”. After the Kremlin dismissed her remarks as “unfounded accusations”, Navalnaya said: “I don’t care how the killer’s press secretary comments on my words. Give Alexei’s body back and let him be buried with dignity. Do not prevent people from saying goodbye to him. “And I really ask all journalists who may still ask questions: don’t ask about me, ask about Alexei.” Lyudmila Navalnaya, his mother, also appealed to the Russian president on Tuesday to release her son’s body so he can be “buried humanely”. She said in a message broadcast on social media: “For a fifth day I cannot see him, they aren’t giving me his body and don’t even tell me where he is. “I appeal to you, Vladimir Putin. Resolving this issue depends on you alone. Let me finally see my son … I demand that Alexei’s body be released immediately so that I can bury him humanely.” Russia’s prison authorities reported on Friday that Navalny felt unwell after a walk and soon became unconscious at the prison in the town of Kharp. An ambulance arrived but he could not be revived, the service claimed, adding that the cause of death was still “being established”. His mother and a lawyer have for four days been unable to retrieve his body from investigators, who said they may not return it for two weeks as they run “tests”. Supporters believe he was murdered, either as an act of foul play or through systematic mistreatment over three years in the Russian prison system. Navalny’s team said Russian law enforcement had also issued a new arrest warrant for Oleg Navalny. The criminal case, which was announced in Russian state media, does not specify what he has been charged with. He is not in the country. Oleg was already on the government’s wanted list for calling on Russians to join his brother’s anti-government protests. He was charged with encouraging violations of Russia’s anti-Covid restrictions on public gatherings. Oleg previously served a three-and-a-half-year prison sentence for fraud in a case where he was tried alongside his brother. Alexei Navalny received a suspended sentence and said his brother’s imprisonment was an attempt to hold him hostage. Navalny’s death in a Russian prison sent a shock wave through opposition circles last week, but some of his closest allies in prison are only learning the news now. In a letter from prison, the opposition member Ilya Yashin wrote that he had only heard of Navalny’s death on Monday. “It’s hard to convey my shock,” wrote Yashin, who had known Navalny for more than a decade. “It’s hard to collect my thoughts. The pain and horror are unbearable.” Yashin was jailed for eight years for publishing reports about the Russian military’s commission of war crimes in Bucha in 2022. He said in letters from prison that he believed Putin had gone “mad from power”. On Tuesday, he compared Navalny’s death to that of the opposition leader Boris Nemtsov, who was gunned down near the Kremlin walls. “Now both my friends are dead,” he said. “I feel a black emptiness inside. And, of course, I understand my own risks. I am behind bars, my life is in Putin’s hands, and it is in danger. But I will continue to push my line.” He said he was sure that Putin had ordered the murder of Navalny behind bars. “For me, there is no question: who killed [Navalny]?” wrote Yashin. “I have no doubt that it was Putin. He’s a war criminal. Navalny was his key opponent in Russia and was hated by the Kremlin. Putin had both motive and opportunity. I am convinced that he ordered the killing.” Navalny supporters reported on Tuesday that Putin had promoted a senior penitentiary official who had been responsible for Navalny’s mistreatment in a prison in the Vladimir region. The deputy director of Russia’s federal penitentiary service, Valery Boyarinev, was given the rank of colonel general, according to a statement signed by Putin. The promotion was a “personal award from Putin for torture and murder”, wrote Ivan Zhdanov, a lawyer and the head of Navalny’s Anti-Corruption Foundation. “Boyarinev personally supervised the torture of Alexei Navalny in prison,” wrote Zhdanov. “Boyarinev personally ordered … the restriction of Alexei’s purchase of food, along with other tortures.” The new US sanctions bill will “hold Russia accountable for what happened to Mr Navalny, and quite frankly, for all its actions over the course of this vicious and brutal war that has now raged on for two years”, Kirby said during a call with reporters. It is timed to the second anniversary of Putin’s full-scale invasion of Ukraine. “One of the most powerful things that we can do right now to stand up to Vladimir Putin, of course, is to, again, pass the bipartisan national security supplemental bill and support Ukraine as they continue to fight bravely and assess their country,” Kirby said. Later on Tuesday the White House national security adviser, Jake Sullivan, said the latest sanctions on Russia will target a range of items, including the country’s defence and industrial bases, along with sources of revenue for the economy.